The present invention relates generally to a fabrication method of an optical element, and more particularly to an evaluation and fabrication method of an optical element having a multilayer film (for example, a multilayer mirror and reflection-type mask). Moreover, the present invention relates to an exposure apparatus having the optical element and a device fabrication method using the exposure apparatus.
Reduction projection exposures using ultraviolet have been conventionally employed to manufacture such a fine semiconductor device as a semiconductor memory and a logic circuit in lithography technology. However, the lithography using the ultraviolet light has the limit to satisfy the rapidly promoting fine processing of a semiconductor device. Therefore, an exposure apparatus using extreme ultraviolet (“EUV”) light with a wavelength of approximately 13.5 nm shorter than that of the ultraviolet (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer very fine circuit patterns of 50 nm or less.
The EUV exposure apparatus uses a reflection-type optical element such as a mirror for an optical system, and a multilayer film that alternately forms or layers two kinds of materials having different optical constants is formed on a surface of the reflection-type optical element. The multilayer film is formed by alternately forming or layering molybdenum (Mo) layer and silicon (Si) layer on a precisely polished glass plate. The thickness of the layer is decided according to the wavelength of the exposure light etc., and for example, a Mo layer is about 3 nm thickness, and a Si layer is about 4 nm thickness. A sum of the thicknesses of two kinds of materials is generally called a coating cycle, which is 7 nm in the above example.
The multilayer mirror reflects EUV light with a specific wavelength when receiving EUV light. Efficiency reflected EUV light is one within a narrow bandwidth around λ that satisfies an interference condition where λ is a wavelength of the reflected EUV light, θ is an incident angle and d is a coating cycle and the bandwidth is about 0.6 to 1 nm. The interference condition is approximately expressible by Bragg's equation (Equation 1), but it shifts minutely from a value obtained from this equation solely due to an influence of refraction in the material etc.2×d×cos θ=λ  (1)
The multilayer mirror in the projection optical system requires very high precision for its surface shape. For example, a permissible figure error σ (rms value) is given in Marechal's equation (Equation 2) below where n is the number of multilayer mirrors in the projection optical system, and λ is a wavelength of the reflected EUV light.
                    σ        =                              λ                          28              ×                              n                                              .                                    (        2        )            
For example, six multilayer mirrors in the projection optical system that uses the exposure light with a wavelength of 13 nm is permitted to have a figure error a of 0.19 nm. The wave front aberration amount is about 0.4 nm for resolution of 30 nm pattern transfer, which is permitted for the whole projection optical system.
Therefore, a surface precision required for the multilayer mirror in the projection optical system is very high, and the surface precision of 0.2 nm as a phase is necessary.
The conventional fabrication method of the multilayer mirror includes a forming process of the multilayer mirror and a shape measuring process.
The multilayer mirror forming process polishes the substrate while repeating the shape measurement with the interferometer that uses visible light, and forms a predetermined shape substrate. Next, the multilayer film is formed on the substrate surface. When actually functioning as the optical system, a best thickness distribution is formed in consideration of the angle and the wavelength of the light irradiated to each position of the multilayer film on the mirror surface.
The shape measuring process measures the surface shape of the multilayer mirror that completes the forming the multilayer film by the interferometer that uses visible light, and judges whether the surface shape of the multilayer film satisfies the predetermined shape (in other words, above figure error σ). The multilayer mirror judged not to have the predetermined surface shape exfoliates the multilayer because the forming the multilayer film has failed, and re-forms the multilayer film.
The method using a Point Diffraction Interferometer (PDI) that directly measures the reflection surface of the multilayer mirror by using the EUV light is known as other prior arts (see, for example, U.S. Patent Application Publication No. 2002/044287 and Japanese Patent Application Publication No. 2000-97620).
Moreover, the method that acquires the layer structure of the X-ray multilayer mirror and the information of interface roughness from the form of X-ray standing wave spectrum is known (see, for example, Japanese Patent Application Publication No. 2002-243669 and Japanese Patent Application Publication No. 2000-55841).
The data concerning the electronic energy loss in the material has been disclosed (see, for example, Youta Nakai et al., “Stopping power of the material to electron of 10 keV or less”, Applied physics volume 51 section 3, page 279, March, 1982). The model calculation concerning the relationship between the reflectivity of the multilayer film and the phase of the reflected light has been disclosed (see, for example, J. H. Underwood and T. W. Barbee, “Layered Synthetic Microstructures as Bragg Diffractors for X-Rays and Extreme Ultraviolet: Theory and Predicted Performance”, Applied Optics 20, 3027 (1981)). Moreover, the photoelectric effect of the multilayer film has been disclosed (see, for example, Michael E. Malinowski, Chip Steinhaus, W. Miles Clift, Leonard E. Klebanoff, Stanley Mrowka, Regina Soufli, “Controlling contamination in Mo/Si multilayer mirrors by Si surface capping modifications”, Proc. SPIE Vol. 4688, Page 442-453, July 2002).
The conventional method measures the surface shape of the multilayer mirror, can not obtain the wave front of the reflected light if the phase is not considered. Therefore, the wave front of the reflected light can not be correctly obtained, and it is difficult to accurately correct the multilayer film.
The PDI method directly measures the reflection surface of the multilayer mirror, and the manufacturing of the apparatus is difficult because the size of the pinhole used to generate an accurate spherical wave is very minute (for example, plural tens nm). Therefore, it is necessary to use very high-luminance light source to introduce enough amount of the EUV light into the minute pinhole, so the measurement system becomes very large and expensive.
Accordingly, it is an exemplary object of the present invention to provide a fabrication method of an optical element that can easily fabricate an optical element that has a multilayer film with a desired performance.